In the manufacture of paper, it has been recognized that the quality of the final product largely depends on the uniformity of the jet of fiber suspension delivered from the headbox to the paper-making machine. Prior attempts to establish a uniform distribution of fibers have involved using a diffuser installed between the inlet and the outlet chambers of the headbox and comprising a cluster of conduits with stepped widenings to promote a turbulent flow. This concept makes the subject of the U.S. Pat. H. Dahl et al., No. 3,725,197 which has been granted on Apr. 3, 1973.
Although this concept constitutes a substantial improvement over the prior art, throughout testing and experimentation, it has been found that this headbox design s not capable of providing optimum results in terms of uniformity of fiber distribution.
Therefore, an object of this invention is an improved headbox with superior fiber distribution characteristics and yet which is simple and inexpensive to manufacture.
The headbox, according to a preferred embodiment of this invention, comprises a diffuser mounted between the inlet and the outlet chambers of the headbox. The diffuser includes a cluster of conduits, each being divided into at least two successive portions of larger cross-sectional area, namely a first portion and a second portion, the second portion being located downstream respectively to the first portion.
In the first portion of the conduit is mounted an elongated stem that defines with an inner wall of the conduit a passage for conveying fiber suspension, the passage, in cross-section, having the shape of a multiply connected domain, such as an annulus.
Preferably, the transition between the first and second portions is abrupt to create vortices in the flow of the fiber suspension.
The above described conduit structure is advantageous for two reasons. Firstly, the multiply connected domain passage allows to obtain an increased contact surface between the fiber suspension and the conduit comparatively to a cylindrical passage, thus increasing the shear forces in the fiber suspension. Secondly, the abrupt transition permits to establish and maintain a controlled level of turbulence. Both factors contribute to break and disperse flocs present in the fiber suspension.
In a preferred embodiment, a stem is provided only in the first portion of the conduit, however, it may be envisaged to construct the stem as a stepped structure of diminishing cross-sectional area in the direction of the fiber suspension flow to define in the conduit a plurality of multiply connected domain passages of increasing dimension.
It has been found advantageous, although not essential, to provide the stem of each conduit of the diffuser with a channel including an inlet portion in fluid communication with the inlet chamber of the headbox, the channel further comprising two or more branches leading from the inlet portion to the fiber suspension passage in the conduit. This arrangement further promotes a turbulent flow because the flow of fiber suspension is split in two or more, and forced to effect a series of sharp bends before entering the fiber suspension passage in the conduit.
In summary, the present invention comprehends a headbox comprising:
an inlet chamber communicating with a source of fiber suspension, the fiber suspension having a direction of flow through the headbox; PA1 an outlet chamber downstream of the inlet chamber, the outlet chamber comprising a discharge opening; PA1 a diffuser mounted between the inlet chamber and the outlet chamber to establish a fluid path from the inlet chamber to the outlet chamber, the diffuser including a plurality of conduits, in fluid communication with the inlet and outlet chambers, each conduit being divided in at least two successive portions, referenced as a first portion and a second portion establishing first and second fiber suspension passages respectively, the first passage having a smaller cross-sectional area than the second passage, each of the conduits including an elongated stem mounted therein, the first passage being defined between the stem and an inner wall of the conduit, the first passage defining in cross-section a multiply connected domain; and PA1 the elongated stem constituting means to create an abrupt enlargement of the cross-sectional area of the conduit at a juncture between the passages in the direction of flow through the headbox, for producing a vortex in the flow of fiber suspension through the conduit. PA1 an inlet chamber communicating with a source of fiber suspension, the fiber suspension having a direction of flow through the headbox; PA1 an outlet chamber downstream of the inlet chamber, the outlet chamber comprising a discharge opening; PA1 a diffuser mounted between the inlet and outlet chambers to establish a fluid path therebetween, the diffuser including a plurality of conduits in fluid communication with the chambers, each conduit including at least two successive portions establishing respective fiber suspension passages of larger cross-sectional area in the direction of fiber suspension flow through the conduit, a stem mounted in each of the conduits, one of the passages being defined between the stem and an inner wall of the conduit, the one of the passages defining in cross-section a multiply connected domain; and PA1 the elongated stem constituting means to create an abrupt enlargement of the cross-sectional area of the conduit at a juncture between the passages in the direction of flow through the headbox, for producing a vortex in the flow of fiber suspension through the conduit. PA1 The invention further comprehends a headbox, comprising: PA1 an inlet chamber communicating with a source of fiber suspension, the fiber suspension having a direction of flow through the headbox; PA1 an outlet chamber downstream of the inlet chamber, the outlet chamber comprising a discharge opening; PA1 a diffuser mounted between the inlet and outlet chambers to establish a fluid path therebetween, the diffuser including a plurality of conduits in fluid communication with the inlet and outlet chambers, each conduit having a predetermined internal volume and including an elongated member received in an initial portion of the conduit, the elongated member filling partially the internal volume to establish in the conduit an initial passage and at least one additional passage downstream of the initial passage, the initial passage having a smaller cross-sectional area than the additional passage, the elongated member constituting means to create an abrupt enlargement of the cross-sectional area of the conduit at a juncture between the passages in the direction of flow through the headbox, for producing a vortex in the flow of fiber suspension through the conduit, the initial passage being defined between the elongated member and an inner wall of the conduit, the initial passage defining in cross-section a multiply connected domain. PA1 an inlet chamber communicating with a source of fiber suspension, the fiber suspension having a direction of flow through the headbox; PA1 an outlet chamber downstream of the inlet chamber comprising a discharge opening; PA1 a diffuser mounted between the inlet and outlet chambers to establish a fluid path therebetween, the diffuser having a plurality of conduits in fluid communication with the inlet and outlet chambers, each conduit having a predetermined internal volume and comprising: PA1 an outlet chamber downstream of the inlet chamber, the outlet chamber comprising a discharge opening; PA1 a diffuser mounted between the inlet chamber and the outlet chamber to establish a fluid path from the inlet chamber to the outlet chamber, the diffuser including a plurality of conduits in fluid communication with the inlet and outlet chambers, each conduit being divided in at least two successive portions, referenced as a first portion and a second portion establishing first and second fiber suspension passages respectively, the first passage having a smaller cross-sectional area than the second passage, each of the conduits including an elongated stem mounted therein, the first passage being defined between the stem and an inner wall of the conduit, the first passage defining a cross-section a multiply connected domain, a section of the conduit coextensive with the stem having a generally constant cross-sectional area throughout its length in absence of the stem; and PA1 an abrupt transition between the passages, the abrupt transition constituting means to create a vortex in the flow of fiber suspension through the conduit.
The invention also extends to a headbox comprising:
Yet, the invention extends to a headbox, comprising:
i) an elongated member mounted in the conduit for filling partially the internal volume to define in the conduit two successive passages of different cross-sectional areas, referenced as a first passage and a second passage, the first passage being defined between the elongated member and an inner wall of the conduit and having a smaller cross-sectional area than the second passage; PA2 ii) between the passages a zone of abrupt transition from one cross-sectional area value to another, the zone of abrupt transition constituting means to create a vortex in the flow of fiber suspension through the conduit; PA2 iii) channel means in the elongated member to establish a fluid communication between the passages and the inlet chamber. PA2 an inlet chamber communicating with a source of fiber suspension, the fiber suspension having a direction of flow through the headbox;
The invention further extends to a headbox comprising: